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New Alpha Centauri planets quest, thanks to TPS !
ngunn
post Oct 17 2012, 09:06 PM
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QUOTE (JRehling @ Oct 17 2012, 06:08 PM) *
it's intriguing to consider whether there is a negative relationship, that inner planets are a sign that planets will not be found at longer periods (~50-500d).


I've just looked through the known multiple planet systems (3+planets) and way over half of them have planets with both under 20d and over 50d periods. It's anecdotal, I suppose, but my impression is that the correlation is positive.
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Reed
post Oct 18 2012, 06:16 AM
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QUOTE (Hungry4info @ Oct 17 2012, 10:00 AM) *
In the paper, they state that they have enough data to detect a 4 Earth-mass planet in the habitable zone of Alf Cen B if it were there.

I don't think this is quite what they said. I assume you are referring to:
QUOTE (Dumusque et al)
However, the observed radial-velocity semi-amplitude is equivalent to the one induced by a four Earth minimum mass planet in the habitable zone of the star (P = 200 days). The HARPS spectrograph has therefore the precision to detect a new category of planets: Habitable super-Earths.

The way I read that, they are only saying HARPS has the sensitivity to detect such a planet. The current detection is based on observing many orbits to identify a periodic signal, and for a habitable zone (200 day) planet they would have far fewer.
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Bill
post Dec 8 2012, 04:36 PM
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I don't know if this is the right thread, but it seems that nobody talked about this future mission selected by ESA which could study Alpha Centauri, among other things.

CHEOPS on ESA website
Cheops Website
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K-P
post Dec 19 2012, 03:11 PM
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It seems we have even more interesting planet system available in our "galactic neighborhood" now.

Science team led by Finnish astronomer Mikko Tuomi found 5 (!) planets around Tau Ceti.

It's getting crowded in this corner of our spiral arm... rolleyes.gif

http://www.space.com/18967-nearby-habitabl...t-tau-ceti.html


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ngunn
post Dec 19 2012, 03:34 PM
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Link to the paper (from BBC News): http://star-www.herts.ac.uk/~hraj/tauceti/paper.pdf
Orbital data are on page 16.
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0101Morpheus
post Dec 20 2012, 12:05 AM
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I've heard about this system last night while lurking on another forum. It's more hard line than this forum though, so I recommend if you want to make an account there you should have a general knowledge about exoplanet mechanics.

http://solar-flux.forumandco.com/

Now I am going to be the skeptic here. While I am not surprised that Tau Ceti could have planets, the inclination of the system is very unknown. Previous studies have given inclinations ranging from near edge on to face on. Now the star is known to have a dust disk and that would normally constrain things, given that low mass systems tend to be coplanar, but there are just as many models of the inclination of the disk as there are the star!

http://adsabs.harvard.edu/abs/2004MNRAS.351L..54G

The RV method only gives minimum masses, if there is a high inclination then these are neptune sized worlds, not super-earths. There are to many to many assumptions right now, and the team used the assumptions that favored their model. There is a better case for Alpha Centauri Bb then these planets found by an experimental method that has yet to be verified.

If I were in charge a followup, I would look for planet b and work my way up. HARPS is in the best position to do this since Tau Ceti is visible in both the North and Southern hemisphere.
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TheAnt
post Jan 5 2013, 06:46 PM
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A bit late perhaps but I happened upon what I think is the original press release for the Tau Ceti system findings.
Nothing new not covered in the other sources you others already have linked though.

University of Hertfordshire news page

The study is part of the European RoPACS program where the same University of Hertfordshire are coordinating the work.

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Paolo
post Mar 27 2015, 09:25 PM
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I bring this old topic back to life for this interesting paper: Hubble Space Telescope search for the transit of the Earth-mass exoplanet Alpha Centauri Bb. Hubble has observed Alpha Centauri and the results are amazing:

QUOTE
We rule out the transiting nature of Alpha Centauri Bb with the orbital parameters published in the literature at 96.6% confidence. We find in our data a single transit-like event that could be associated to another Earth-size planet in the system, on a longer period orbit.
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algorimancer
post Apr 1 2015, 05:39 PM
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QUOTE (Paolo @ Mar 27 2015, 04:25 PM) *

"Wow" seems a bit inadequate. I look forward to a replication. Looks very plausibly like an Earth-size planet, but the inferred short orbital period (15 days) suggests Mercury-like temperatures, even given the lower luminosity of Alpha Centauri B (intuitively; I didn't do the math). Anyone know what "MCMC" refers to in this context? In my world, it's "Markov Chain Monte Carlo", but that doesn't make sense to me in this context [EDIT: Checked the reference... turns-out is is indeed a Markov Chain Monte Carlo... interesting].
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TheAnt
post Apr 2 2015, 09:54 AM
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@algorimancer: Mercury temperature yes, in that ballpark.
I had a look, and hum, they publish on one observation only. So could you tell how you think they apply the Monte Carlo probability distribution on a sample of one? Because I don't get it either.
(If I did a study of mine with just a sample of one, I'd be hanged upside down from the nearest flagpole as an example of ridicule. Astronomers must live in a relatively protected world compared to some of us others. tongue.gif )
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centsworth_II
post Apr 2 2015, 10:52 AM
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QUOTE (TheAnt @ Apr 2 2015, 05:54 AM) *
...they publish on one observation only.
The information age! Giving "outsiders" a look at the process of science, not just the final product. (An optimistic interpretation.)


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algorimancer
post Apr 2 2015, 01:55 PM
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QUOTE (TheAnt @ Apr 2 2015, 03:54 AM) *
@algorimancer: Mercury temperature yes, in that ballpark.
...Monte Carlo probability distribution on a sample of one? Because I don't get it either.

My best guess is that they use MCMC to model the light curves, with standard errors, and demonstrate that the "detection" curve differs significantly from the background curve, after accounting for other variables which affect the outcome. Frankly, I'm aware of MCMC methods, but don't use them personally. My loose understanding at the moment is that they are a handy way of estimating and putting error bounds on predictor variables, but I haven't taken the time to study how they're used in the current context. I'm not a Bayesian, so for similar problems I'd draw from a different bag of tricks.

The authors don't seem to be over-emphasizing the detection, and the light curve of the detection event is visually persuasive (which doesn't mean it is real). The comparison of two continuously sampled light curves, a linear reference versus the event curve, is arguably rather more robust than comparing a single event point to a reference point, so perhaps we can allow a little leeway on the sample-of-one thing, in an exploratory context. Of course, nobody will believe it until it has been reliably replicated.
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algorimancer
post Apr 2 2015, 02:57 PM
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Expanding on my earlier thoughts regarding the MCMC method used in the paper ... a component of MCMC is the Markov chain (MCMC: Markov Chain Monte Carlo). A Markov chain is essentially a means of describing the probabilities of changing states among 2 or more states, which in this case might model changing from the neutral state with no eclipse (no planet blocking the star) to the detection state of an eclipse (planet blocking the star), and back again, so a Markov chain is clearly applicable in this case. The Monte Carlo component would involve simulation of a process with an underlying randomization based upon the known underlying variability; the authors invested quite a bit of effort in quantifying the variability of their observations after factoring-out the non-random components, so this would feed into the process. So, the combination of components in an MCMC might simply be used to put error bars on the transition probabilities associated with the Markov chain as a basis for demonstrating that a state change (detection) occurred. On the other hand, the combination of tools encompassed in an MCMC is used for other purposes, like summing a multidimensional integral or finding estimates of predictor variables in a model, which is why I'm a bit unclear on how it was actually used here.

I can envision several other means of testing for the presence of an eclipse. The simplest would be to partition the samples of the light curve over small intervals, and do something like a t-test comparing the mean intensity (or frequency) prior-to versus during a candidate event. Another would expand on that notion, modeling each partition with a non-linear model, and test for differences between the curves. Yet another would be to to do a running regression of the intensity to a mathematical model of eclipse light curves (I like this approach). I'm not sure there is a single best method for quantifying a detection, but I'd guess that the people who do this for a living have given it rather a lot of thought smile.gif

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TheAnt
post Apr 3 2015, 08:58 AM
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Thank you algorimancer for your long reply, I finally get it.
Though I had a vague notion that they might have treated the ingress and egress separately. There's nothing in my world like a synthetic set of data to compare with, where the astronomers certainly know what a planet transit should look like.
But yes a second detection is required, and if that's done by one other team we finally have gotten a start on revealing what's hidden in the Alpha Centauri system. =)

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Aldebaran
post Apr 10 2015, 11:40 PM
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QUOTE (Bill @ Dec 8 2012, 04:36 PM) *
I don't know if this is the right thread, but it seems that nobody talked about this future mission selected by ESA which could study Alpha Centauri, among other things.

CHEOPS on ESA website
Cheops Website



From the Cheops website:

"These key parameters will help scientists to understand the formation of planets from a few times the mass of the Earth – ‘super-Earths’ – up to Neptune-sized worlds."

It seems to be a mix of parameters and makes the statement vaguer than it ought to be. One moment, we're talking mass, and then we're talking "size". Size is quite vague, but not normally applied to mass. In terms of radius, Neptune is about 3.9 times the size of the Earth. In terms of volume, it's 58 times the size of the Earth.

(and in terms of surface gravity, Neptune is about the same as the Earth. )

It all comes down to communication with the general public.
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